The invention relates to a method for converting the signal modulation of channels of an optical multiplex system to subcarrier frequencies.
Optical microwave and millimetric wave transmission (the latter is incorporated below in general into the term microwave transmission) is of great interest, inter alia, for supplying base stations in mobile telephone systems with high carrier frequencies. The aim in this case is to obtain the complete microwave signal, which contains the microwave subcarrier and a signal modulation, which is generally of a substantially lower frequency, directly from the photodiode. Various methods are known for optical microwave generation, which are represented, for example, in the overview by R.-P. Braun and co-authors in Nachrichtentechnik/Elektronik, Berlin, 45 (1995) pages 63-67, entitled xe2x80x9cOptische Mikrowellentechnik fxc3xcr zukxc3xcnftige zellulare Mobilfunknetzexe2x80x9d [xe2x80x9cOptical Microwave Engineering For Future Cellular Mobile Radio Networksxe2x80x9d].
In the heterodyne methods, the generation of the microwaves as differential frequency of two light-waves of which one is modulated and the other is a local or remote oscillator is performed in the photodiode. A disadvantage is the phase noise produced. This is avoided in the self-heterodyne method. Alternatives are, for example, the resonance modulation of mode-coupled semiconductor lasers, or the sideband-injection locking of two laser diodes.
Another solution is specified in the reference entitled xe2x80x9cMillimeter-Wave Electro-Optical Upconverter For Wireless Digital Communicationsxe2x80x9d by J. Park, M. S. Shakouri, and K. Y. Lau, Electronics Letters, 31 (1995), pages 1085 to 1086. Signal modulation is firstly carried out in the laser diode, followed by upward conversion by modulation with the subcarrier frequency.
Time-division multiplex, wavelength division multiplex, code division multiplex and polarization division multiplex are generally known for the transition from simplex transmission to an optical multiplex transmission system, with subcarrier division multiplex, in addition, being known for microwave transmission.
It is accordingly an object of the invention to provide a method for converting the signal modulation of channels of an optical multiplex system to subcarrier frequencies which overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which different signals in the baseband or intermediate frequency band, which have been impressed on optical carriers of different wavelengths, are converted with the aid of a component onto a microwave or millimetric wave subcarrier. It is assumed that the useful signal frequencies are sufficiently lower than the subcarrier frequency.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for converting signal modulation of optical channels to one of microwave and millimetric wave subcarriers, which includes:
impressing in each case a signal modulation having a frequency lower than a subcarrier frequency on optical sources having different emission wavelengths;
bring together a plurality of optical wavelength channels thus formed into a fiber bus;
modulating the plurality of optical wavelength channels with a modulator controlled by one of the subcarrier frequency and a subharmonic thereof; and
separating the plurality of optical wavelength channels in a wavelength-selective fashion and leading them to direct receivers.
It is essential to the invention that all the wavelength channels brought together are converted, when modulated, to higher frequencies by a single external modulator having a high cutoff frequency. In this case, no distortions of the signal modulation already performed occur, but only harmonics of this modulation. They permit the external modulator to be operated in the case of subharmonics of the subcarrier frequency. If desired, all the wavelength channels can be amplified by a single fiber amplifier. Intensity-modulated signals are required for the purpose of direct reception, for example by photodiodes, for which reason consideration is firstly given to an intensity modulator for converting signals onto the microwave subcarrier.
By contrast, when use is made of a phase modulator and dispersion elements in accordance with the reference written by W. Nowak and M. Sauer entitled xe2x80x9cDynamikbereichserhxc3x6hung in optischen Mikrowellen-Subcarriersystemen durch Dispersionsmanagementxe2x80x9d [xe2x80x9cDynamic Range Expansion In Optical Microwave Subcarrier Systems By Dispersion Managementxe2x80x9d], MIOP ""97, Sindelfingen, April 1997, it is possible to achieve at the photodiode a power gain of 3 dB optical corresponding to 6 dB electrical. Since a phase modulator needs only half the drive power by contrast with a comparable intensity modulator, the result is a power gain of 9 dB when the ratio of photodiode power to drive power of the modulator is considered.
In using the method of the invention with a standard fiber as the dispersion element in the 1.55 xcexcm band in the case of a subcarrier frequency of 60 GHz, this requires a fiber length of approximately 1 km (or approximately 3 km, 5 km, . . . ) (the subcarrier power going sinusoidally with the fiber length). If the fiber lengths of all the wavelength channels differ from one another by no more than +/xe2x88x92200 m, it is then possible to make use in the bus of a single additional fiber length which supplements the existing mean fiber length to 1 km, additional losses for the maximum and minimum fiber lengths can then be  less than 0.5 dB electric. Additional lengths are to be provided in the individual connections in the case of large differences in length.
The occurrence of harmonics of the subcarrier modulation frequency owing to the nonlinear modulator characteristic can lead to crosstalk from one optical channel to other ones, when the optical channel spacings correspond to integral multiples of the subcarrier frequency. This can be countered by selecting the optical channel spacings suitably such that higher harmonics of other channels do not fall into the subcarrier band.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for converting the signal modulation of channels of an optical multiplex system to subcarrier frequencies, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.